AI Gateway silently ignores mask parameter for OpenAI inpainting (gpt-image-1)

[mttzzz]

Description

When using generateImage() with @ai-sdk/gateway and providing a mask parameter for OpenAI inpainting (gpt-image-1), the Gateway silently ignores the mask and generates a completely new image instead of editing only the masked region.

The same image + mask + prompt sent directly to OpenAI's /v1/images/edits endpoint produces correct inpainting — only the masked area is regenerated while unmasked areas are preserved.

Reproduction

import { gateway } from '@ai-sdk/gateway';
import { generateImage } from 'ai';

const result = await generateImage({
  model: gateway.imageModel('openai/gpt-image-1'),
  prompt: {
    text: 'Premium food photography of a gourmet burger...',
    images: [sourceImageUint8Array],
    mask: maskUint8Array, // RGBA PNG, transparent=edit, opaque=preserve
  },
  size: '1024x1536',
  providerOptions: {
    openai: { quality: 'high' },
  },
});

Evidence

Setup:

• Source image: 1024×1536 PNG photo of a burger with a branded flag on top
• Mask: 1024×1536 RGBA PNG, ~15.7% transparent (ellipse over the burger area), rest opaque (preserve flag, fries, background)

Via Gateway — mask completely ignored, entire image regenerated from scratch:

• The flag, fries, background, composition — everything is different
• No warnings or errors returned

Via direct OpenAI API — mask correctly applied:

const formData = new FormData();
formData.append('model', 'gpt-image-1');
formData.append('image', imageFile);
formData.append('mask', maskFile);
formData.append('prompt', prompt);
formData.append('size', '1024x1536');
formData.append('quality', 'high');

await fetch('https://api.openai.com/v1/images/edits', {
  method: 'POST',
  headers: { Authorization: `Bearer ${apiKey}` },
  body: formData,
});

• Only the burger (masked area) was regenerated
• Flag, fries, background — preserved from original

Analysis

I traced the code through the SDK:

1. ai core (generate-image.ts → normalizePrompt()): Correctly extracts mask from the prompt object
2. @ai-sdk/gateway client (index.js ~line 1030): Correctly encodes mask via maybeEncodeImageFile() and includes it in the JSON body sent to the Gateway server:

   ...mask && { mask: maybeEncodeImageFile(mask) }

3. Gateway server: Receives the mask in the request body but appears to route to OpenAI's /v1/images/generations instead of /v1/images/edits, silently dropping the mask parameter.

Server-side logs confirm the mask reaches generateImage():

Prompt type: object, keys: text,images,mask

Expected Behavior

When mask is provided in the prompt, the Gateway should:

1. Detect that this is an inpainting request (image + mask present)
2. Route to OpenAI's /v1/images/edits endpoint (not /v1/images/generations)
3. Send the request as multipart/form-data with image and mask files (as required by OpenAI's edits API)

Actual Behavior

The Gateway ignores the mask parameter entirely and generates a completely new image as if no mask was provided. No warning or error is returned.

Versions

• ai: 6.0.158
• @ai-sdk/gateway: 3.0.95
• Node/Bun: Bun 1.3.11
• OS: Windows (production: Linux)

Additional context

This is related to how OpenAI's image API has two separate endpoints:

• /v1/images/generations — text-to-image (no mask support)
• /v1/images/edits — inpainting (requires image + mask as multipart files)

The Gateway needs to dispatch to the correct endpoint based on whether a mask is present in the request.

[Zelys-DFKH]

Gateway routes to /v1/images/generations even when a mask is present. Inpainting needs /v1/images/edits. The fix is to branch the dispatch logic on mask presence before the request goes out. I can send a PR for that.

[Zelys-DFKH]

After tracing it more carefully, the dispatch to /v1/images/edits happens on the Gateway server, not in the SDK — the client already sends the mask correctly (gateway-image-model.ts:92). This needs a fix on Vercel's backend, not in this repo. Sorry for the noise.

[No5972]

After tracing it more carefully, the dispatch to /v1/images/edits happens on the Gateway server, not in the SDK — the client already sends the mask correctly (gateway-image-model.ts:92). This needs a fix on Vercel's backend, not in this repo. Sorry for the noise.

The endpoint of /v1/images/edits returns an error 404 of "The requested resource was not found", while /v1/images/generations does not. I wonder if Vercel hasn't thought of implementing this API endpoint yet.

[No5972]

After tracing it more carefully, the dispatch to /v1/images/edits happens on the Gateway server, not in the SDK — the client already sends the mask correctly (gateway-image-model.ts:92). This needs a fix on Vercel's backend, not in this repo. Sorry for the noise.

The endpoint of /v1/images/edits returns an error 404 of "The requested resource was not found", while /v1/images/generations does not. I wonder if Vercel hasn't thought of implementing this API endpoint yet.

There are different kinds of image generation models or services. I guess it takes a matters of time for them to adapt different kinds of image generation services.

[lgrammel]

Identify issue type

{
  "evidence": [
    "Using generateImage() with @ai-sdk/gateway and OpenAI gpt-image-1 inpainting, a provided mask is reportedly ignored and a completely new image is generated.",
    "The same image, mask, and prompt sent directly to OpenAI /v1/images/edits works correctly, preserving unmasked regions.",
    "Reporter traced that the SDK client includes the mask, but the Gateway server appears to route to /v1/images/generations or lacks /v1/images/edits support.",
    "A commenter confirms /v1/images/edits through the Gateway returns 404 while /v1/images/generations works."
  ],
  "rationale": "The issue describes a concrete current behavior failure: inpainting mask input is silently ignored or unsupported through AI Gateway despite being sent by the SDK, producing incorrect output instead of masked editing.",
  "type": "bug",
  "confidence": "high"
}

[Zelys-DFKH]

See above: the issue is in the Gateway server, not in this repo.